Effect of Hormones on Glucose, Glycogen and Lipid Metabolism − Rakyat Biologi

These notes are intended to be read in conjunction with the other handouts and a biochemistry textbook, particularly Berg, Tymoczko & Stryer, Biochemistry 5^th Edition: Chapters 10, 15, 21, 22 and 30. Numbers in brackets in this text refer to page numbers in Stryer. Most of the diagrams in the presentations are taken from Stryer (as indicated by the Page, Figure or Table Number).

Hormones (see also BS1060 notes on this topic)

These are chemical messengers that coordinate the activities of different cells in multicellular organisms. Hormones are synthesised and secreted in specific endocrine cells (which may be organised into an endocrine gland) are released into the bloodstream where they travel to a specific site of action where they have specific effect on the physiological and biochemical activities of the target cell (or target organ). The concentrations of hormones in circulation are closely regulated primarily via changes in the rate of synthesis and secretion by the endocrine cell. They are rapidly metabolised to inactive products ensuring that the systems can respond rapidly to physiological requirements. The concentrations of hormones in circulation are generally very low (10^-12 M to 10 ^–8M are common) and their effects are mediated by rapid changes in hormone concentration (10-fold to 100-fold changes are fairly typical) which are amplified at the target cell by a signal transduction mechanism converting the hormonal signal into a larger biochemical effect. The concentrations of hormones in circulation are monitored by a specific hormone receptor protein which is found only in the target cell, usually on the plasma membrane (or in the cell nucleus in the case of steroid and thyroid hormones), and which initiates a series of responses within the target cell.

An example of a signal transduction mechanism is the effect of glucagon on cyclic 3’ 5’AMP (cAMP) in the liver and of adrenaline (known as epinephrine in USA) on cAMP in muscle cells. cAMP was the first example of a second messenger to be discovered. Second messengers carry hormonal signals inside cells when the hormone itself (the first messenger) is unable to cross the plasma membrane. (396)

Formation of cAMP- a second messenger (Fig 21.14)

cAMP is formed as the result of binding of glucagon (587) (or adrenaline/epinephrine) to the specific hormone receptor on the outside of the liver cell (or muscle in the case of adrenaline), this leads to the activation of a G-protein (a heterotrimeric protein which binds GTP in its active form and which has a GTPase activity which inactivates it). This acts both as a switch and a timer, which in turn, activates adenylate cyclase at the cytoplasmic face of the plasma membrane. This is an example of a signal transduction mechanism, involving three different membrane-bound proteins, which generates a signal inside the cell (cAMP) proportional to the concentration of hormone outside the cell. (398-401)

Adenylate cyclase (Fig 15.8) (401) is responsible for the synthesis of cAMP via the cyclization of the a phosphoryl group of ATP yielding pyrophosphate as a by-product. The activity of this enzyme, even in the activated form is very low, hence the cAMP concentration in the stimulated cell is in the 10 –30 mM range, as compared to ATP which is about 5 - 10 mM.

cAMP phosphodiesterase (588) which hydrolyzes the 3’ phosphate bond of cAMP and breaks down the second messenger into an inactive product (AMP). The activity of this enzyme also acts as a switch and a timer that acts to terminate the signal. This enzyme may also be activated by the hormone, resulting in only a very rapid, transient increase in cAMP.

An increase in extracellular hormone thus results in a rapid (within a few seconds) increase in the second messenger which remains elevated for a matter of a few minutes, this time is dependent on the rate of hydrolysis of cAMP. Thus a transient hormonal signal a transient hormonal signal lasting only a few seconds is amplified and prolonged by the generation of the second messenger.

How does the change in cAMP lead to a metabolic response? (Fig 21.14)

All known effects of cAMP inside the mammalian cell is mediated by the activation of cAMP – dependent protein kinase (called PKA) (403). There are many other protein kinases called PKB, PKC, PKD etc., which respond to different second messengers and other signalling cues. PKA (Fig 10.28) is an enzyme that has two regulatory (R) subunits and two catalytic (C) subunits and is inactive in the absence of cAMP. When the cAMP level inside the target cell rises, the second messenger binds to the R subunits causing the dissociation of the complex and the release of the active C-subunits. A lowering of cAMP results in the reassociation of the R and C subunits and the inhibition of kinase activity.

PKA is a protein kinase (Fig 10.28) (276-279) which means that it can catalyse the transfer the terminal (g) phosphoryl residue from ATP to phosphorylate other proteins at specific serine and threonine residues. This is known as a covalent modification of the protein (Fig 30.2)(847). This can result in a stable change (i.e. remains stable if you purify the protein, unlike activity changes due to metabolites) in activity of the phosphorylated protein (either an activation or inactivation) which can occur within seconds or minutes.

Protein phosphorylation is readily reversed by Protein Phosphatases (a family of related enzymes) which hydrolyze the covalently-bound phosphate causing the restoration of the original activity state of the protein (277).

A range of key regulatory enzymes are regulated by hormones by this method, including Glycogen Synthase, Pyruvate Kinase (Fig 16.21), PFK2/FBPase2 (Figs 16.19, 16.20), Phosphorylase Kinase (Fig 21.14) and Hormone-sensitive Lipase(Fig 30.13). The effect of the activation/inhibition of these enzymes by covalent modification by cAMP and PKA is an increased breakdown of glycogen and inhibition of glycolysis in the liver leading to an elevation of blood glucose (hyperglycaemia). Increased lipolysis of triacylglycerol (Fig 30.13) leads to an increase in Free Fatty Acids (FFA).

Glycogen Metabolism (Chapt. 21)

Glycogen is a highly branched polymer of glucose units linked by a-1,4 glycosidic bonds with a-1,6 branch points every 8-12 glucose residues (Fig 21-1, Fig 21.15). This means that the polymer forms a large molecular weight, insoluble granule with branches into the cytosol. The enzymes involved in the synthesis and degradation of the polymer are found in the cytosol associated with the non-reducing ends of the many branches on a single glycogen molecule. This allows the storage in the liver of large amounts of glucose (Fig 21.2) in a chemically inert form that can be synthesised or degraded very rapidly in response to the requirements of the body for glucose at any one time (851-58). In the fed condition when insulin is secreted into the bloodstream, the conversion of glucose to glycogen is increased. During fasting the level of insulin is lowered and glucagon is secreted and as a result there is an increased release of glucose from glycogen to buffer the blood glucose. It is important that the blood glucose is maintained within fairly narrow limits (4 – 6 mM). In hypoglycaemia (arterial blood glucose < 2 mM) brain function is compromised, persistent fasting hyperglycaemia (glucose 8 – 20 mM) as found in diabetes mellitus can have devastating long-term consequences – degeneration of the retina, kidney and nerve damage (858).

Glycogen Synthesis and Degradation

Glycogen synthesis and degradation involve a different set of enzymes to allow the separate regulation of the two pathways. The key enzymes of glycogen synthesis are UDP-glucose pyrophosphorylase and Glycogen Synthase (589-90) whilst the key regulatory enzyme of glycogen degradation is (Glycogen) Phosphorylase (579) (Fig 21.4).

Phosphorylase is regulated by both allosteric effectors, metabolites, which signal the energy state of the cell such as ATP, AMP and glucose 6-P, and by reversible protein phosphorylation which is regulated by hormones such as insulin, glucagon and adrenaline. (Figs 21.11 and 21.12)

Phosphorylase exists in two forms (a and b) a phosphorylated a form (Ser¹⁴–O-PO₃) which is very active and a dephosphorylated b form (Ser¹⁴–O-H) which is much less active. The enzyme is converted into the active form by phosphorylase kinase, a specific protein kinase which phosphorylates the enzyme at Ser¹⁴ (Fig 21.13). In turn, this enzyme, is itself activated by protein phosphorylation by cAMP-dependent protein kinase (PKA) (which in turn is activated by glucagon - see above and page 586-88). The signal transduction cascade is shown on page 595 in Fig 23-14 except that the hormone shown is adrenaline (epinephrine) rather than glucagon. Phosphorylase a can be converted back to the inactive form by the action of protein phosphatase 1 which is activated by insulin via a different signal transduction pathway (593-594).

Glycogen synthesis involves UDPG pyrophosphorylase which synthesises UDP-Glucose, an ‘activated’ form of glucose which is used by Glycogen Synthase.

Glycogen Synthase (589-590) again is regulated by covalent modification, except that in this case it is inactivated by the phosphorylation at different serine residues by a number of different protein kinases (including PKA) and activated by dephosphorylation by protein phosphatase.

The net effect of these regulatory properties is closely coordinated. Insulin and glucagon are the most important regulators of hepatic glucose metabolism. Insulin stimulates the storage of metabolic fuels (including glycogen) while glucagon promotes glycogen degradation in the liver. (854-56).

Fatty acid Metabolism (Chapt. 22)

Fatty acids have a number of different functions in the cell, including their role as fuel molecules, which are an alternative to glucose as source of energy in the cell. Fatty acids can be stored in large amounts in adipocytes (fat cells) (Fig 22.1) as esters of glycerol in the form of triacylglycerol (TAG) - also known as triglycerides (602). Fatty acids are long-chain alkyl carboxylic acids which vary in chain length but the most common ones have 16 or 18 carbons and may be saturated or unsaturated with one or more double bonds.

Triacylglycerols are highly concentrated stores of metabolic energy which yield about 9 kcal/g of energy when completely oxidised as compared to a yield of about 4 kcal/g for protein or carbohydrate metabolism. An average man has about 135,000 kcal stored as TAG, the equivalent of about 40-50 days energy requirements. TAGs are rather insoluble compounds, which are transported in the blood associated with proteins in complexes known as lipoproteins (e.g LDL (Fig 26.16) or VLDL, low density or very low density lipoproteins or chylomicrons {Fig 22.5})

Triacylglycerol Lipase

The initial event in the utilization of fat is the hydrolysis of TAG by lipase yielding Free Fatty Acids (FFA) and glycerol. The latter is readily converted to glycolytic/gluconeogenic intermediates by glycerol kinase. The activity of TAG lipase (605) is regulated by hormones, in particular by glucagon, adrenaline or noradrenaline, which increase cAMP in the adipocyte by a mechanism similar to that described above in the liver. Again PKA becomes activated and this protein kinase phosphorylates and thus activates this lipase. Insulin inhibits this activation. The result is that in the fasting state FFA in circulation increase from about 0.2 mM to about 1mM in prolonged starvation. FFA tend to be rather toxic and insoluble and they are transported in the bloodstream to their site of metabolism as a complex with an abundant protein called albumin which acts as a transport protein.

As with glycogen, fatty acids are synthesised and degraded by different pathways (Fig 22.2):

Mitochondrial Uptake of Fatty Acids

Fatty acids are degraded in mitochondria by the oxidation of the Acyl CoA (a thioester of the fatty acid) at the b-carbon yielding Acetyl CoA which can be oxidized by the citric acid cycle. Before the can be b-oxidized, the medium chain length FFA are converted into the fattyacyl CoA by acyl CoA synthase in the mitochondrial outer membrane (606). This cannot cross the inner mitochondrial membrane, so the needs to be converted to Acyl carnitine (Fig 22.8) which can be shuttled across the membrane by a translocase, fattyacyl CoA is re-formed on the matrix side of the inner mitochondrial membrane and the carnitine can then cross back to the cytosolic side to pick up the next acyl unit (607).

b-Oxidation of Fatty Acids - a reminder (Fig 22.8, 22.9 Table 22.1)

The acyl CoA is degraded by a cycle of 4 recurring reactions which shortens the fatty acid by 2 carbons in each cycle yielding Acetyl CoA and FattyAcyl CoA which is two carbons shorter. The reactions catalysed are shown in Fig 22-8 basically involve oxidation, hydration, a further oxidation and thiolysis catalysed by the following enzymes:

· Acyl CoA dehydrogenase

· Enoyl CoA hydratase

· 3-Hydroxyacyl CoA dehydrogenase

· b-ketothiolase

The degradation of a C₁₆ fatty acid such as palmitoyl CoA requires 7 reaction cycles and generates 8 Acetyl CoA, and 7 molecules of both FADH₂ and NADH. All of these are energy-rich and can be used to generate ATP in the mitochondria (BS1090 notes, Chapt 18). The complete b-oxidation of Palmitic acid can generate 106 ATP molecules!

Generation of Ketone Bodies by the Liver

The Acetyl CoA formed in fatty acid oxidation can only be oxidised completely to CO₂ if there is a balance of carbohydrate and fat metabolism, i.e. there should be enough oxaloacetate available in the mitochondria to react with Acetyl CoA formed by b-oxidation. Remember that oxaloacetate is normally formed from pyruvate, a glycolytic intermediate. In fasting or in the diabetic state, oxaloacetate is used for gluconeogenesis and is not available for the TCA cycle. Under these conditions Acetyl CoA undergoes alternative reactions (Fig 22-20, Fig 30.17) leading to the generation of ketone bodies, acetoacetate, b-hydroxybutyrate and acetone (616). The plasma level of fatty acids and ketone bodies increase in starvation (Fig 30.16) (Table 30.2) . Abnormally high levels of ketone bodies are also present in the blood of untreated Type 1 diabetics (those suffering from Insulin-Dependent Diabetes Mellitus, IDDM) and these volatile molecules may be detected in the breath of a sufferer. In prolonged starvation, the ketone body concentration in the blood can reach 5 mM and they can be used by muscle, and eventually after several weeks, by the brain, as an alternative metabolic fuel to glucose (856-58). In some tissues, such as heart muscle, ketone bodies are the normal respiratory fuel in preference to glucose (616) (Fig 30.18)

Regulation of b-Oxidation of Fatty Acids

The regulation of b-oxidation appears to be regulated not by hormones acting directly on the enzymes involved (e.g covalent modification) but rather indirectly via the provision of substrates (FFA, ketone bodies, pyruvate) which can be metabolised by mitochondria. That is not to say that mitochondrial enzymes cannot be regulated by phosphorylation / dephosphorylation. You may recall that pyruvate dehydrogenase which generates Acetyl CoA from pyruvate is regulated by a specific protein kinase and phosphatase (467) (Fig 17.4). This dehydrogenase catalyses a key irreversible step in animals because they are unable to convert Acetyl CoA (and therefore FFA) to glucose. Fatty acids are not gluconeogenic in animals but they are in plants!

Fatty Acid Synthesis

Fatty acid synthesis involves a not only a different set of enzymes to those involved in b-oxidation, but also a different subcellular location i.e. the cytosol. The intermediates involved are covalently linked to the SH group of acyl carrier protein rather than CoA-SH. In contrast to the enzymes of b-oxidation, the catalytic sites involved in Fatty Acid synthesis in mammals are located in a single protein called Fatty Acid Synthase. The growing fatty acid chain is elongated by 2-carbon units derived from malonyl CoA and the reductant required is NADPH rather than NADH and FADH₂ which are generated in b-oxidation.

Stoichiometry of Fatty Acid Synthesis (Chapt. 22.4.6, page 622)

Transfer of Acetyl CoA to the Cytosol (Fig 22.25) - role of citrate

Acetyl CoA Carboxylase

This is a key enzyme in fatty acid biosynthesis in that it catalyses an irreversible reaction – the ATP dependent carboxylation of Acetyl CoA yielding malonyl CoA (617). The reaction involves a carboxybiotin-enzyme intermediate. The inactive form of the enzyme is an octamer of 265 k_Da but polymerises into an active filamentous form in the presence of citrate (Fig 22.28) . You will recall that citrate levels are a good indicator of the availability of Acetyl CoA and ATP. The Acetyl CoA could be derived from carbohydrate, via pyruvate, so that carbohydrate can be converted to fat.

Fatty Acid Synthase

Fatty acids are synthesised in eukaryotes by a multifunctional enzyme complex (a dimer of 260 k_Da in mammals) with 7 different catalytic sites on a single polypeptide chain. (617-622). Effectively a 2-carbon unit from malonyl CoA is condensed with acetyl CoA and CO₂ is released. The 4-carbon unit formed undergoes NADPH dependent reduction reactions before condensing with a further 2-carbon unit from malonyl CoA to yield a 6-carbon unit. The fatty acid molecule is built from 1 Acetyl CoA and 7 Malonyl CoA until it is a 16-carbon unit which remains attached to the enzyme until it is released as the 16-carbon fatty acid palmitate. Details of the reaction are shown on pages 618-620 in Stryer.

NADPH Supply

Fatty acid synthesis therefore requires a plentiful supply of reductant in the form of NADPH. Some of this is generated when Acetyl CoA and oxaloacetate are transferred from mitochondria to cytosol and, at the expense of ATP hydrolysis, catalysed by ATP-citrate lyase, the oxaloacetate formed is converted to malate which is then used to generate NADPH by NADP⁺ - linked malate enzyme (malic enzyme) (623)(Fig 22-25). This cycle provides only some of the NADPH required for fatty acid synthesis; the remaining NADPH is generated (564-4) (Fig 20-20) by glucose 6-phosphate dehydrogenase and 6-phosphogluconate dehydrogenase – yet another instance where fatty acid synthesis is linked to the supply of carbohydrate.

Regulation of Fatty Acid Synthesis (a.k.a. lipogenesis)

The prime regulatory step in fatty acid synthesis is the provision of Malonyl CoA by acetyl CoA carboxylase. The carboxylase is controlled by the availability of substrates and by the levels of insulin, glucagon and adrenaline. Insulin stimulates fatty acid synthesis by activating the carboxylase whereas glucagon and adrenaline have the reverse effect. Citrate, an indicator that the molecules required for fatty acid synthesis are available, is an allosteric activator of the carboxylase, whilst palmitoyl CoA, the product of Fatty Acid Synthase is an inhibitor. (624-625)

Acetyl CoA carboxylase is regulated by covalent modification by an AMP-activated protein kinase (not PKA) which phosphorylates the protein at a single serine residue and converts the enzyme into an inactive form Fig 22.27, Fig 22.26). This kinase is activated by AMP and inhibited by ATP and therefore its activity reflects the energy status of the cell. The phosphorylation is reversed and the carboxylase activated by the action of Protein Phosphatase 2A. This latter enzyme is inhibited by glucagon and adrenaline via PKA and is activated by insulin by an unknown mechanism. The net effect of all these changes is that the hormonal control of Acetyl-CoA carboxylase is rather similar to that of glycogen synthase. Fatty acids are synthesised when food is plentiful and metabolized in times of starvation. (626 and 851-54)

These are all short-term regulatory mechanisms but in addition there are long-term mechanisms which involve specific gene transcription. Animals that are fasted and then fed high carbohydrate, low fat diets show increases in the amounts of Acetyl CoA carboxylase, Fatty acid Synthase, Glucose 6-Phosphate dehydrogenase over a few days which are reflected in a general increase in lipogenesis.

Triacylglycerol can be synthesised de novo from carbohydrate in the liver, exported in the form of a lipoprotein (VLDL) and broken down by a lipoprotein lipase in the blood capillaries in the adipose tissue. The fatty acids can then be taken up by adipose tissue and the triacylglycerol resynthesised and stored in the adipocytes. Deposition of triacylgycerol is stimulated by insulin via the activation of lipoprotein lipase.

Metabolic Integration

There are a number of general points which can be made from these lectures on glycogen and fatty acid metabolism (and indeed previous lectures on glycolysis, gluconeogenesis and citric acid cycle) (845-47)

· ATP is the universal currency of energy in the cell

· AMP signals an energy deficit in the cell

· ATP is generated by the oxidation of glucose, fatty acids and amino acids

· ATP is required for biosynthetic reactions

· NADPH is the major electron donor for reductive biosynthesis

· Biomolecules are constructed from a relatively restricted set of simple molecules

· Biosynthetic and degradative pathways are distinct – there are key regulatory enzymes which are unique to either synthesis or degradation.

· Control of synthetic and degradative pathways is closely integrated

· Key enzymes are usually regulated by allosteric interactions with important cellular metabolites

· The activities of key enzymes are often controlled by covalent modification, usually phosphorylation, which is initiated by changes in extracellular hormones

· Changes due to covalent modification are transient lasting a few sec or min.

· Hormones and other factors, such as dietary components can alter the rate gene transcription and therefore the rate of synthesis of key enzymes thus altering the amounts (and therefore the activities) of these enzymes within the cell. These changes take hours or days.

Different tissues have different metabolic functions (851-54) but each function is linked and integrated:

· Brain is highly dependent on glucose except in long-term starvation

· Muscle can use glucose, fatty acids or ketone bodies but preferentially uses stored glycogen for bursts of activity via anaerobic glycolysis producing lactate

· Heart muscle is dependent on ketone bodies as a fuel source

· Adipose tissue is specialized for the storage and release of fatty acids as required by the other tissues

· Liver plays a central role in the provision of fuel substrates to all these other tissues via:

· Gluconeogenesis from lactate and amino acids

· Glycogen synthesis and degradation

· Fatty acid synthesis and their export in the form of VLDL

· Ketone boy formation

· The main function of glycolysis is to provide precursors for biosynthesis

Diabetes Mellitus

Hormones play an important part in the integration of metabolism which becomes very apparent when insulin secretion is diminished or absent as in IDDM (see above, Page 5) while glucagon secretion remains normal. Diabetes mellitus can also be induced in experimental animals by treatment with alloxan or streptozotocin, which destroy the b-cells and therefore inhibit insulin secretion giving rise to the following symptoms:

[1]       High Fasting Blood Glucose (6-10 mM)
[2]       Low Liver Glycogen Content
[3]       Increased Hepatic Gluconeogenesis
[4]       Decreased Hepatic Glycolysis
[5]       Muscle Protein Mobilized - amino acids used for gluconeogenesis

[6] Triacylglycerol Mobilized From Adipose Tissue -

Free Fatty Acids generated give rise to Ketone Bodies (acetone and acetoacetone)

These effects are analogous to what happens in the starved state when the effects of glucagon predominate over the effects of insulin. In starvation a major function of the liver is to provide the brain with a constant supply of glucose which is essential for the latter organ. However in the diabetic the symptoms arise despite an adequate food intake because the tissues fail to respond in an appropriate manner as the result of the lack of insulin. In both starved and diabetic state there is a high level of cyclic AMP (cAMP) in the liver. Insulin can be regarded as a signal that the animal has been FED recently while glucagon (and therefore cAMP) is a HUNGER signal. (854-858)

Insulin administration corrects the abnormalities listed above.

i.e. it lowers blood glucose by increasing glucose oxidation, decreasing gluconeogenesis, stimulating liver and muscle glycogen synthesis while inhibiting glycogen breakdown. It also reverses the effects on protein and fat metabolism causing increased deposition of these energy reserves in muscle and adipose tissue, respectively.

D R Davies BS1090 Notes 2004